Fiber optic cables are used to transmit telephone, television, and computer data information in indoor and outdoor environments in non-multiplexed and multiplexed optical transmission systems. In wave division multiplexed systems, optical performance characteristics play a significant role in maintaining high data rate transmission.
Optical attenuation, the loss in transmitted power, and chromatic dispersion, the differential transit time at adjacent wavelengths, are examples of optical performance characteristics in such transmission systems. Optical attenuation is typically due to absorption, scattering, and leakage of light from the waveguide and is customarily measured in a fiber, or cable, as a loss value in dB/km. Chromatic dispersion in fiber optic waveguides can be viewed as the sum of material and waveguide dispersions. Changes in refractive index with wavelength give rise to material dispersion. In bulk glass (silica) fibers, material dispersion increases with wavelength over a wavelength range of about 0.9 μm to 1.6 μm. Material dispersion can have a negative or a positive sign depending on the wavelength. Waveguide dispersion results from light traveling in both the core and cladding of an optical fiber. Waveguide dispersion is also a function of wavelength and the refractive index profile. Wavelength and material dispersion affects combine to yield an overall positive or negative chromatic dispersion characteristic at any given point in a given optical fiber. Optical performance concerns regarding pulse spreading caused by chromatic dispersion have created a need for dispersion compensating systems. Dispersion compensating systems employing, for example, positive and negative dispersion compensating fibers, are nevertheless subject to the optical performance constraints associated with optical attenuation.
A fiber optic cable design that acknowledges chromatic dispersion affects is described in U.S. Pat. No. 5,611,016. The patent pertains to a dispersion-balanced optical cable for reducing four-photon mixing in wave division multiplexing systems, the cable being designed to reduce cumulative dispersion to near zero. The dispersion-balanced optical cable requires positive and negative dispersion fibers in the same cable. Further, the positive dispersion aspect includes a dispersion characteristic defined as the average of the absolute magnitudes of the dispersions of the positive dispersion fibers exceeding 0.8 ps/nm.km at a source wavelength. In addition, the negative dispersion fiber characteristic requires the average of the absolute magnitudes of the dispersions of the negative dispersion fibers to exceed 0.8 ps/nm.km at the source wavelength. The aforementioned optical fibers are ribbonized, single-mode fibers designed for the transmission of optical signals in the 1550 nm wavelength region. The fibers are non-stranded or non-helically enclosed within a mono-tube cable, and are described as having an attenuation at 1550 nm of 0.22-0.25 dB/km, and attenuation at 1310 nm of <0.50 dB/km. At defined parameters, the positive-dispersion characteristic is described as being +2.3 ps/nm.km and the negative-dispersion characteristic is described as being −1.6 ps/nm.km.
Other patents describe optical performance characteristics relating to a time division, rather than wave division, system. For example, U.S. Pat. No. 4,478,488 describes selective time compression and time delay of optical signals, without discussing the problems associated with attenuation or chromatic dispersion. A system is described using discrete channels having a dispersive section coupled to a standard multi-waveguide transmission section, and then another dispersive section. Signals are intended to propagate spatially out of phase, which can minimize channel coupling phenomena. An embodiment requires respective plastic coatings formed on twisted optical fibers, the coatings having varying diameters for varying the helix of the fibers in the cable. Individual fibers are spaced from the axis of the twist by different distances. This causes some fibers to twist more than others and extends the length of fiber located at the outside of the bundle compared to one nearer the inside of the bundle. Using a multicore cable made up of cores embedded in a single cladding, each fiber is fixed at a helix that is different than the helix of any other fiber.